C and G are frequently mutated into T and A in coding regions of human genes.

Nucleotide mutations in human genes have long been a hot subject for study because some of them may lead to severe human diseases. Understanding the general mutational process and evolutionary trend of human genes could help answer such questions as why certain diseases occur and what challenges we face in protecting human health. In this study, we conducted statistics on 89,895 single-nucleotide variations identified in coding regions of 18,339 human genes. The results show that C and G are frequently mutated into T and A in human genes. C/G (C or G)-to-T/A mutations lead to reduction of hydrogen bonds in double-stranded DNA because C-G and T-A base pairs are maintained by three and two hydrogen bonds respectively. C-to-T and G-to-A mutations occur predominantly in human genes because they not only reduce hydrogen bonds but also belong to transition mutation. Reduction of hydrogen bonds could reduce energy consumption not only in separating double strands of mutated DNA for transcription and replication but also in disrupting stem-loop structure of mutated mRNA for translation. It is thus considered that to reduce hydrogen bonds (and thus to reduce energy consumption in gene expression) is one of the driving forces for nucleotide mutation. Moreover, codon mutation is positively correlated to its content, suggesting that most mutations are not targeted on changing any specific codons (amino acids) but are merely for reducing hydrogen bonds. Our study provides an example of utilizing single-nucleotide variation data to infer evolutionary trend of human genes, which can be referenced to conduct similar studies in other organisms.